10.1117/2.1200707.0801

Ophthalmic Shack-Hartmannwavefront sensor applicationsDaniel Neal

Eye aberration measurements can be used in wavefront-guided laserablation and clinical research applications.

Shack-Hartmann wavefront sensors (SHWFSs) have been usedfor a wide variety of applications over a period spanning morethan 35 years, with human-eye measurements being by far themost common in terms of the number of sensors in routine use.For instance, these sensors have become the norm for supportinglaser refractive surgery, while being increasingly used in variousophthalmology and optometry applications.

The architecture of a Shack-Hartmann aberrometer is simi-lar to that of a laser guide star arrangement. Light is projectedinto the eye and scattered from the retina. It is then collected bythe SHWFS to analyze eye aberrations (see Figure 1). The aber-rations are readily categorized as either lower or higher ordereffects, with defocus and astigmatism being the primary lowerorders, and coma, trefoil, spherical and other aberrations beingthe higher orders. A convenient measure of eye aberration is thediopter value, a unit of measurement of the refractive power ofa lens, equal to the reciprocal of the focal length: the higher thenumber, the higher the eye aberration. The eye can be extremelyaberrated, with defocus ranging from −16 to +8 diopters, andcylinder ranging up to 5–6 diopters.

It is common practice to use a Keplerian telescope with ad-justable focus to optically correct the defocus term (sphericalequivalent). This is done using a one degree of freedom closed-loop adaptive optics system to minimize the SHWFS error. Ina few seconds, the instrument can find the appropriate defocuscondition and neutralize it optically, so that the sensor only mea-sures the cylinder and higher order aberrations. Some commer-cially available systems use a fixed optometer, and others alsocorrect for the astigmatism terms.1

The instruments are accurate to a fraction of a wave, have adynamic range of 50–70µm, and can measure an eye in a fewseconds. To date, there are approximately ten different compa-nies manufacturing such instruments for various markets.

There are currently four basic clinical applications using oc-

Figure 1. Arrangement for guide star measurement of the eye. SLD:superluminescent diode.

ular measurement systems: wavefront guided ablation in laserrefractive surgery; auto-refraction for spectacle and contact lensfitting; diagnostics of kerataconus, ectasia or other aberratedconditions; and research in accommodation, scattering, tear-film,customized contacts, and other applications.

Wavefront-guided ablation has now become the standardtechnique for nearly all forms of laser refractive surgery. Priorto the introduction of the wavefront instruments, laser assistedin situ keratomileusis (LASIK) and photorefractive keratectomy(PRK) procedures had the drawback of inducing a significantamount of (mostly spherical) higher order aberrations that wentundetected, except through variations in manifest refraction asa function of pupil size. With the wavefront-guided treatment,aberrations are now measured directly and can provide the in-formation needed to adjust the nomograms for optimizing thesurgery. This approach has been very successful over the last 6–7 years with the result that the majority of procedures are nowfully customized.

Figure 2 shows an example of the same wavefront measure-ment analyzed for two different pupil sizes. Results are indica-tive of significantly different refractions with a sphere value of

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+0.28D for the 4mm pupil and of −1.31D for the 5mm pupil.These values point to a large spherical aberration componentand the patient in this case may be best served by prescribingtwo pairs of spectacles: one for normal daytime vision and theother for night-time use, particularly for driving. Alternatively,some form of customized correction may be used.

One major application of this technology is the identificationand tracking of unusual pathologies, including common condi-tions such as kerataconus, ectasia, corneal scars, dry-eye anddamage resulting from injury. Some of these, notably kerata-conus, can be detected earlier in a wavefront measurement thanwhen using other techniques such as corneal topography.

The ocular wavefront measurement instrument also repre-sents a key element in understanding human eye function. It canprovide insights in how the perceptions of a subject change withaberrations, in eye accommodation processes, and in the dynam-ics of eye-movement-induced aberrations. Furthermore, it canalso quantify many of these effects, all of which are critical to ad-vancing human vision knowledge. For example, instruments formeasuring accommodation are starting to provide quantitativefeedback on the effectiveness of some newly emerging surgicaltechniques.

As the general population ages, understanding and correctingfor presbyopia is also becoming an increasingly important issue.Numerous corrective techniques are available, with the morecommon being multi-focal contact lenses, intraocular lenses,laser refractive surgery, and scleral surgery. To assess their re-spective efficiency, it becomes crucial to use a technique that canmeasure objective changes in the lens, and to distinguish this ef-fect from multi-focal, depth-of-field, and neurological contribu-tions. To this end, we have developed a sequence of instrumentsdedicated to accommodation measurements. Using the wave-

Figure 2. Variation in-eye refraction as a function of pupil size. Left:4mm-pupil. Right: 5mm-pupil.

Figure 3. Accommodation measurement with an objective aberrometer.Left: Aberrometer with external dynamic binocular target. Right: Ex-ample data showing both defocus response along with change in coma.

front sensor in this context involves the careful presentation oftargets to the subject while simultaneously recording the full dy-namic ocular response. The sensor has proven ideal since it pro-vides a means, inherent in the data, for distinguishing betweenthe various effects. Figure 3 shows one of these instruments andan example measurement.

The development of ocular Shack-Hartmann measurementsystems has improved our ability to make detailed measure-ments of the eye. These advances have also improved techniquesused for correcting the optical errors of the eye, including laserrefractive surgery, contact lenses, spectacles, and contact lens fit-tings.

Author Information

Daniel NealAdvanced Medical Optics and Wavefront Sciences, Inc.Albuquerque, New Mexico

Daniel Neal has developed many applications of wavefrontsensing technology. These include astronomical applications,super-Mach wind tunnel testing, and silicon wafer metrology.He is a co-founder of WaveFront Sciences, a leader in wave-front technology. WaveFront was recently acquired by AdvancedMedical Optics.

References

1. D. R. Neal, Shack-Hartmann sensor engineered for commercial measurement applica-tions, R. Shannon and R. Shack (eds.), Legends of Applied Optics, SPIE Press, 2005.ISBN: 9780819458445

c© 2007 SPIE